Transponders, which are also referred to as tags or labels, are well known in the art and are designed to communicate with a reader, which is also known as a base station. Usually, the reader sends a signal to the transponder. If the transponder is close enough to the reader, then the transponder receives this signal and sends a response signal to the reader.
International application for patent No. 99/60510 discloses a system for contactless identification of a plurality of transponders located in a communication volume defined by an electromagnetic field coming from a reader. When the reader wants to investigate if and which transponders are in its range, the reader sends a query signal, usually known as an inventory command. The inventory command comprises information about a plurality of reply windows (time slots), in which the transponders are supposed to reply to the inventory command. In response to the inventory command, each of the transponders selects a reply window (time slot) during which it transmits its reply signal. The transponders randomly choose one of the time slots for transmitting their reply signals. Thus, collisions may occur if at least two transponders reply during the same time slot. Then, a conventional reader discards the reply signals and sends a new inventory command until it receives unambiguous reply signals from the transponders. This procedure, however, is relatively time consuming.